Apparatus for sweeping acoustic mines



Aug. 1, 1961 J. c. WATSON APPARATUS FOR SWEEPING ACOUSTIC MINES Filed April 27, 1955 FIG. I

INVENTOR John 6. Watson BY )7 z ATTORNEYS nited States Patent fice Patented Aug. 1, 1961 The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to underwater noisemaking and more particularly to apparatus for the underwater generation of low frequency sound and a method for employing the apparatus in the sweeping of acoustic responsive mines.

The sweeping of some acoustic minm requires high intensity sounds of low frequency, i.e. below 50 cycles per second. Mechanical low frequencynoisemakers have had considerable use and are eifective within their limitations. However, they become quite large and expensive when made to functionas desired in addition to being very difiicult to stream and recover, and due to their cost they cannot very well be considered to be expendable. In addition, the problem of supply at distant harbors, or other sweeping areas, is large because of the many replacement parts needed as wellas the special cables and electrical equipment required. Other types of noisemakers which have proven practical include those employing as the energy sourcethe explosion of solids, but these types have been found to be too expensive for routine sweeping.

In accordance with the present invention a rapid-burning solid of high thermal output is suitably packaged for controlled burning under water and preferably within a confining case such that the contact between the sea water and the burning solid generates steam in sufficient quantities to produce geyser-like action resulting in cavitation in the adjacent medium. The natural repetition rate of this geyser-like action depends somewhat upon the structure, shape, and dimensions of the combustible solid, the spacing of the casing around the solid, as well as the hydraulic inductance between the device and open water.

An object of the present invention is to provide simple, inexpensive means for the generation of low frequency noise of the high intensity required for sweeping acoustic mines. 4

Another object of the invention is the provision of a combustion type underwater nois'emaker utilizing discrete units so that amplitude modulation can be controlled to some extent.

A further object of the invention is to provide a method for sweeping acoustic mines with discrete sound sources.

Other objects and advantages of the invention as well as its method of use will be understood from the following description when read in connection with the accompanying drawing in which:

FIG. 1 is a vertical cross-section showing the preferred form of the invention;

FIG. 2 is also a side elevation partly in section of another embodiment the invention may take; and

FIG. 3 is a plan view of an igniting packet.

As shown in FIG. 1 the noise generating device includes a combustible solid 10 which may be a pyrotechniccapable of burning underwater. The analysis of one suitable combustible solid being substantially as follows:

Percent Magnesium, mesh 48 Barium nitrate 21 Sodium nitrate 21 Sodium oxycelate 5 Wax 3 Linseed oil 1 Castor oil 1 A combustible solid of this composition delivers 1.4 1() calories per gram and a cylinder thereof fifteen inches long and four inches in diameter will burn for about two minutes. These pyrotechnics as presently procured for aerial flares are enclosed in a cardboard casing fitted at one end with a wooden plug and having an open end to which is secured a black powder squib for igniting the flare. This cardboard casing serves the useful purpose of controlling the burning from one end to the other. When the combustible solid itself, including its cardboard case, is burned underwater it generates useful sound and may be employed in practicing the method of the invention. However, when so used free in the water, the level of its sound output is too low to sweep an area large enough for practical naval operations. Considerably higher sound output is obtained when the combustible unit is mounted in spaced relation within an open-end housing such as a pipe provided at one end with an intake flapper valve and open at its other end such as is shown in FIG. 1. As illustrated in FIG. 1, one end of a cylinder of a combustible solid 10 is fitted with a wooden block 11 and the two are tightly sheathed with a slow burning material 12, such as cardboard. This sheathed assembly is suitably mounted in spaced concentric relation to a tube 13, as by screws 14 sunk into the wood block 11 through the Wall of the tube 113 and by set screws 15 threaded through the wall of the tube 13 and pressed into the cardboard casing 12 to hold the assembly firmly in position. The wooden base is spaced from the end of the tube 13 provided with an intake flapper valve 16 by a distance somewhat less than the diameter of the flapper valve 16 so as to make certain that the valve 16 does not open to a position where it will not reseat under pressure of the generated energy. The tube 13 towards its open end extends beyond the free end of the combustible solid 10 by a distance which is not critical but which is preferred to be in the order a of the diameter of the tube 13. I The free end of the combustible solid 10 has secured thereto, as by staples 18, an igniting pad 17 containing a fuse 20 and an easily ignited substance such as black powder 21. As best shown in FIG. 3, this pad 17 may comprise a fabric bag for confining the powder 21 to close proximity with the fuse 20. A protruding end of the fuse 20 may be provided with a black powder squib 19 which contains an electric heating element 24 supplied by leads 22 with electric current from any suitable source not shown.

In the embodiment shown in FIG. 2 the end of the pipe casing 33 containing a flapper valve 36 is provided with a pump impeller 40 adapted to be driven by a fractional horsepower electric motor 41 which may be powered by a sea water battery 42. The motor 41 and its associated equipment is suitably mounted as by bolts 43 in a compartment defined by a housing 44 provided with a plurality of inlet ports 45 for supplying the pump impeller 40 with water which it discharges through the flapper valve 36 into the tube 33 at a rate greater than it would flow by hydrostatic pressure alone, whereby the quantity of water which is turned into steam is increased with a resulting increase in acoustic output. The particular embodiment shown in FIG. 2. is adapted to operate with the open end of its pipe casing 33 directed downwardly and for this purpose a gas filled chamber 46 is secured to the motor compartment housing 44 to provide suflrcient buoyancy for maintaining the equipment in this desired vertical position without, however, giving the assembly positive buoyancy, because it is desired to have the noisemaker sink rapidly towards the bottom for the purpose of approaching any mines which may be thereon. This purpose will be appreciated when it is remembered that the amplitude of the sound is attenuated proportionally to a power of the distance it travels.

In its use for minesweeping purposes the squib 19 is electrically ignited through the leads 22 to start the fuse 20 in the packet 17 burning which in turn ignites the combustible solid at which time the complete assembly is dropped overboard where it continues burning while sinking rapidly towards the bottom. The number of assemblies dropped at any one time or the interval between releases will depend upon the tactical situation of the area and the nature of the mine mechanisms to be swept, as determined by intelligence groups. With this method of operation it will be evident that any vessel capable of navigating the water of the area to be swept becomes an acoustic minesweeper merely by loading on board the necessary number of the units shown in FIGS. 1 or 2. Thus, small boats with practically no risk to themselves can be used in sweeping operations and expensive larger and more vulnerable craft need not be involved. This method of operation makes possible, especially in friendly waters, the employment of a mother ship for logistic support of a fleet of small boats engaged in acoustic sweeping so that operating areas may be considerably extended with the additional real benefit of making certain that all areas are swept without unnecessary overlapping, since the mother ship can direct or vector the sweeping boats as well as have sufficient navigating equipment aboard for this purpose. In this manner it becomes possible for a channel of considerable length to be completely swept of acoustic mines in what might be considered one pass. It will, of course, be obvious that the acoustic devices of the invention may be launched from aircraft.

It is quite usual for acoustic mines to be so constructed that for actuation they require an acoustic signal which increases in amplitude at a rather definite rate. For

sweeping mines having this type of mechanism the acoustic sources of the invention are launched in increasing numbers in accordance with time, e.g., starting with one device at unit time the number launched is doubled every three seconds for a total time of nine or twelve seconds, constituting a sweeping cycle, this process being repeated 4 at spaced intervals as determined by intelligence groups to be required to sweep the particular mines thought or found to be in the area. These requirements include ship counting mines, that is, mines requiring a certain number of actuating signals before their firing circuit is closed.

From the above description of the preferred embodiments thereof it will be evident that the present invention makes available acoustic sweeping means in the form of small discrete units readily handled manually, and versatile enough to make available various new methods of minesweeping. While for the purpose of describing the invention only two embodiments thereof have been disclosed, it is to be understood that other embodiments incorporating the invention may take various forms without departing from the spirit of the invention nor the scope of the appended claims. Also, the methods of sweeping employing the sound sources of the invention which have been described are not limiting since other methods will readily occur to those skilled in the art to meet the unique conditions which will be encountered in practice. It will also be understood, since no two areas to be swept are ever identical, that detailed information could not be given in advance as to the best method to be employed. Factors involved in determining the minesweeping tactics most likely to succeed in any one area include the depth of the water, oceanographic and hydrographic features which influence acoustic propagation, the type of the bottom, and above all the mine mechanism which must be actuated if the sweeping operation is to succeed.

Whenever it appears to be necessary to make the initial pass into mined waters it should be made in the deepest part of the area to decrease the risk of a dangerous acoustic field being present under the sweeping vessel.

What is claimed is:

1. A device for generating low frequency underwater sound comprising an elongated tubular chamber having one end open and the other end closed, an underwater combustible solid mounted concentrically in and spaced from the walls and the ends of said chamber, and an intake flapper valve mounted in the closed end of said chamber for admitting water therethrough, whereby the burning of the combustible solid when the device is submerged will generate steam which expels water from said chamber to produce cavitation in the adjacent water.

2. A device in accordance with claim 1 and wherein a housing defining a freeflooding compartment having the closed end of the elongated tubular chamber as one of its walls is provided with pump means for discharging water through the flapper valve mounted in said closed end.

3. A device in accordance with claim 2 wherein buoyancy means secured to the freeflooding compartment function to cause the device to sink through the water with its open end directed downwardly.

References Cited in the file of this patent FOREIGN PATENTS 612,452 Great Britain Nov. 12, 1948 

